Acceptable levels of distortion in the mains supply system

Harmonic voltage distortion

In the mains supply system, harmonic voltage distortion is the consequence of the flow of harmonic currents through the impedances in the power supply circuit connected to the converter. A typical power supply system at an industrial or mining plant consists of a source of AC power generation, which can either be a local generating station in a small system or a power station at the other end of a transmission line or transformer in a large system.

The impedance between the ‘ideal’ generator and the main busbar is usually referred to as the source impedance Zs of the supply system.

Additional impedance, usually comprising cables, busbars, transformer, etc exists between the main busbar and the converter busbar and is the cable impedance Zc, as shown in Figure 1 below.

The flow of current to a variable speed motor is controlled by the converter. The current is non-sinusoidal due to the non-linearity of the converter and the generation of harmonic currents. The flow of distorted current through the power distribution and supply system produces a distorted volt drop across the source and distribution impedances in series.

Other equipment, such as electric motors or even other consumers can be connected to the main busbar. Consequently, this busbar is referred to as the point of common coupling (PCC).

The voltage at the PCC will be distorted to an extent depending on the magnitude of the distorted current, the magnitude of the impedances and the ratio between them.

The distribution impedance must be calculated from the design details of the distribution system.

A high distribution impedance will tend to reduce the voltage at the point of common coupling but increase it at the converter connection terminals. This voltage distortion can cause interference with the electronic trigger circuits of the converter and give rise to other problems if it becomes too high.

If the magnitude and the frequency of each harmonic current is known, a simple application of Ohm’s law will give the magnitude of each harmonic voltage and the sum of them will give the total distorted voltage.

Total harmonic distortion (THD) of voltage and current

From AS 2279-1991 Part 2, the total harmonic distortion (THD) of voltage and current are given by the following formulae. Generally, it is sufficient to use values of n up to 25.

Where:

• V_T = Total harmonic voltage distortion
• I_T = Total harmonic current distortion
• V₁ = Fundamental voltage at 50 Hz
• I₁ = Fundamental current at 50 Hz
• V_n = nth harmonic voltage
• I_n = nth harmonic current

The acceptable levels of harmonics in industrial power supply networks are clearly defined in Table 1 of the Australian standard AS 2279-1991 Part 2: disturbances in mains supply networks.

Reference // Practical Variable Speed Drives and Power Electronics – Malcolm Barnes CPEng, BSc(ElecEng), MSEE, Automated Control Systems, Perth, Australia (Get this book from Amazon)